The invention relates to a wear distance sensor for a brake pad of a friction brake.
Vehicles and certain technical devices, for example, hoists, frequently use friction brakes to convert kinetic energy. Disc brakes are preferred in this case, especially in passenger automobiles and in the case of commercial vehicles. In a typical construction, these disc brakes consist of a brake caliper, two brake pads, and the brake disc. Closing forces are applied and braking forces are absorbed by means of the brake caliper. The closing forces act via both brake pads on the brake disc, which experiences a deceleration of its rotational movement as a function of the level of the closing force. This deceleration is decisively also determined by the coefficient of friction between brake disc and brake pad. Since the pads are constructively designed as wear parts and the coefficients of friction are dependent on the strength, the pads are generally softer than the brake disc, i.e., the pads experience a change of the pad thickness over their usage duration, that is they wear.
The necessity results from this pad thickness change, which is caused by the wear, that it is recognized by the vehicle user when the pads have reached a so-called remaining pad thickness or wear limit and a replacement of the brake pads is necessary. In order to ensure optimum use of the brake pads, it is desirable for the instantaneous actual state of the individual brake pads, i.e., their pad thickness, to be able to be detected at any time.
Proposals for this purpose have been made, one of which provides regularly checking a marking applied to the brake pad and thus providing a decision criterion for the pad change. However, this check requires a certain regularity and certain cleaning work in order to unambiguously recognize the marking.
Another proposal provides a continuous wear display, in the case of which the wear of the brake pads of a wheel brake is indirectly measured via an electronic rotational angle measurement on a wear adjuster integrated in the wheel brake. It is disadvantageous in this case that the wear of the brake disc is also incorporated and therefore a summation wear detection of the system of brake pads/brake disc occurs.
DE 10 2007 008 729 B4 describes a sampling element for detecting a wear distance of a brake pad, in particular of a friction brake. It includes a housing having a sampling side and a terminal side; and at least one electrical conductor in combination with at least one electrical resistor element. Two dimensions of the resistor element and the electrical conductor are not constant in dependence on the wear distance. The resistor element is attached to a mount, which has a first conductor and a second conductor having a respective terminal for the electrical connection to the resistor element. The mount is designed as a wire bow, and the first and second conductors are respectively connected, at an end opposite to the terminals, to a conductor bridge.
It is therefore an object of the present invention to provide an improved wear distance sensor of a brake pad. A further object comprises providing a brake pad having such a wear distance sensor.
This and other objects are achieved by a wear distance sensor having, and a brake pad having the wear distance sensor, for detecting a wear distance of a brake pad, in particular of a friction brake. The wear distance sensor includes a housing having a friction side and a terminal side, and a probe having two electrical conductor elements in combination with at least one electrical resistor element. Two dimensions of the resistor element are not constant in dependence on the wear distance. A third dimension of the resistor element, which is implemented as three-dimensional, is not constant in dependence on the wear distance. The resistor element extends as a three-dimensional body in the direction of the wear distance with a resistor length and perpendicularly thereto with a resistor width, and the resistor element extends perpendicularly to this surface thus formed in a resistor thickness. The resistor element has a triangular shape in projection in the plane which is formed by the resistor length and the resistor width, wherein the base of this triangle extends in the direction of the resistor width and the legs of this triangle are connected in a tip which is arranged on the friction side. The first conductor element is applied in an extension in a connection section on the resistor element in the direction of one leg of the triangle in a first terminal section and is electrically conductively connected to the resistor element and extends up to the tip of the triangle. The second conductor element is electrically conductively connected at an edge to the resistor element with a connecting end opposite to the first conductor element in a second terminal section.
A basic concept of the invention comprises providing a wear distance sensor having a three-dimensional electrical resistor element having a spatial formation, of which three dimensions are changeable in dependence on the wear distance of a brake pad.
It is thus advantageously achieved that a resistance of the resistor element is changed in dependence on the wear distance, wherein this resistance forms a measure of the wear distance.
Further advantages of the invention are as follows:
(1) direct measurement of the wear state of a brake pad, wherein the actual pad thickness is detected;
(2) permanent measurement of the wear state, the actual pad thickness is output at all times;
(3) the pad wear can be separately detected and output for each pad;
(4) cost savings are possible by way of decreased mechanical structure and lower material use;
(5) optimization of the use of the pad wear volume, because the disc wear is not incorporated in the case of this measurement;
(6) dimensions of the sampling element are freely changeable; and
(7) integration of a temperature detection is possible because of the property that the coefficient of resistance of the resistor element is temperature-dependent.
The wear distance sensor has a probe in a housing, which is arranged in a brake pad such that it is subjected to the same wear as the brake pad itself. The wear distance sensor having the probe is therefore also shortened in the same amount by the wear. This advantageously results in a resistance change, which is simple to measure and which is continuously provided as an ohmic resistance value, i.e. a measurement, and therefore display of the pad thickness of each brake pad is possible independently of the wear of the associated brake disc.
A wear distance sensor according to the invention for detecting a wear distance of a brake pad, in particular of a friction brake, having a housing having a friction side and a terminal side; and a probe having two electrical conductor elements in combination with at least one three-dimensional electrical resistor element, wherein two dimensions of the resistor element are not constant in dependence on the wear distance, is characterized in that a third dimension of the resistor element, which is implemented as three-dimensional, is not constant in dependence on the wear distance.
The context that the electrical resistance of a conductor is dependent on its specific resistance, its length, and its cross section is advantageously utilized in this case.
The wear distance sensor has the resistor element as a three-dimensional body, which extends in the direction of the wear distance with a resistor length and perpendicularly thereto with a resistor width. In addition, the resistor element extends perpendicularly to this surface thus formed in a resistor thickness. Therefore, three variables which can be fixed beforehand are provided, using which the resistor element is adaptable to the requirements of a large measurement effect.
For this purpose, the resistor element can have a total resistance which is changed in the event of a decreased resistor length. For example, the total resistance can be decreased. This can be achieved simply in that the resistor thickness of the resistor element decreases in the event of decreased resistor length and the resistor width increases in the event of decreased resistor length. Of course, the total resistance can also increase in the reverse manner.
Alternatively or in combination therewith, the resistor thickness of the resistor element can change continuously or in steps in the event of decreased resistor length, i.e., decrease or increase, wherein the resistor width can decrease or increase continuously or in steps in the event of decreased resistor length.
In one embodiment, it is provided that the resistor element has a triangular shape in projection in the plane which is formed by the resistor length and the resistor width, wherein the base of this triangle extends in the direction of the resistor width and the legs of this triangle are connected in a tip which is arranged on the friction side.
In a preferred design, the first conductor element is applied in an extension in a connection section on the resistor element in the direction of one leg of the triangle in a first terminal section and is electrically conductively connected to the resistor element and extends up to the tip of the triangle, wherein the second conductor element is electrically conductively connected at the edge to the resistor element with a connecting end opposite to the first conductor element in a second terminal section. This connection section ensures that an electron distribution is implemented as substantially homogeneously as possible in the resistor element. This current is tapped using the connecting end of the second conductor element, which functions as the current tap. The second conductor element does not extend up to the tip of the probe. It would form a contact at the tip with the conductor end of the connection section of the first conductor element, which is not desirable, because, for example, a measuring current would thus have to be increased unnecessarily and protective measures would have to be taken. In addition, an auxiliary energy source would thus be overloaded or would have to be oversized.
In a further design, the legs of the triangle can be implemented in the form of a predefinable curve or a plurality of predefinable curves. A characteristic curve of the resistor element which is particularly capable of adaptation is thus to be achieved.
In an alternative embodiment, the resistor element can have a trapezoidal shape in projection in the plane which is formed by the resistor length and the resistor width, wherein the base and the bottom side of this trapezoid extend in the direction of the resistor width and the base of this trapezoid is arranged on the friction side. Also in this case, similarly as in the case of the triangle shape, the first conductor element is applied in an extension in a connection section on the resistor element in the direction of one leg of the trapezoid in a first terminal section and is electrically conductively connected to the resistor element and extends up to the base of the trapezoid, wherein the second conductor element is electrically conductively connected to the resistor element with a connecting end opposite to the first conductor element in a second terminal section on the bottom side of the trapezoid.
Also in this embodiment, the legs of the trapezoid can be implemented in the form of a predefinable curve or a plurality of predefinable curves.
The resistor element can be a metal film or can be used as a resistor layer as a thin-film or thick-film system. A carrier element is provided as the resistor layer, which accommodates and stabilizes the resistor layer.
The construction of the probe of the wear distance sensor can be manufactured easily in industrial mass production, e.g., for producing electronic circuit boards or resistor components (potentiometers, measuring resistors, resistor layers on substrates (ceramics, inter alia)).
The resistor element is formed from a material which has a high specific resistance characteristic value and simultaneously a low coefficient of temperature.
It is provided that the housing completely encloses the resistor element, and is preferably made of an insulating material, e.g., ceramic, high-temperature filler, or plastic, whereby the advantage is provided that the resistor element is enveloped in an electrically-insulated, heat-resistant, and corrosion-resistant manner and can be easily processed.
In a further embodiment, the wear distance sensor has a voltage divider having at least one first divider resistor and at least one second divider resistor. The voltage divider allows a standard signal, e.g., 0 to 5 V, to be generated for the pad wear. Therefore, for example, complex external analysis electronics can be reduced or even omitted entirely. An absolute temperature detection can also be implemented by the use of this voltage divider. In addition, a temperature compensation of the characteristic curve of the sensor may be implemented. The actual pad wear state can therefore be detected independently of the pad temperature.
In a further embodiment, it is provided that the at least one second divider resistor is electrically connected in parallel to the resistor element.
In still a further embodiment, the at least one first divider resistor is electrically connected in series to the at least one second divider resistor and the resistor element and is electrically conductively connected to a third conductor terminal end.
In another embodiment, the at least one first divider resistor can be a temperature-dependent resistor. A separate temperature measurement is therefore possible.
In still a further embodiment, the housing comprises a probe housing section having the friction side and a fastening section, which is connected thereto, having the terminal side and having a fastening profile. It is advantageous if the fastening profile is designed for a press fit, whereby the sensor is secured in a brake pad carrier against axial displacement.
In a further embodiment, the housing has at least one shoulder as a stop surface for defining a fixed position as the reference position.
A brake pad has the above-described wear distance sensor. Of course, it is also possible that a plurality of wear distance sensors can be used in a brake pad.